In a fuel cell system using reformed gas produced from a hydrocarbon reforming material as fuel, the reformed gas is purified to decrease the carbon monoxide (CO) in the reformed gas. The purification of reformed gas is carried out by a purifier such as a hydrogen purifier with a hydrogen permeable membrane and a selective oxidation part incorporated in the reformer. To avoid complication of the fuel cell system entailed by incorporation of such a purifier, JP05-299105 discloses a fuel cell with a hydrogen permeable metal layer on the electrolyte layer in which unpurified reformed gas can be used as fuel.
However, when hydrogen permeable metal is exposed to hydrogen at low temperatures, the hydrogen permeable metal absorbs the hydrogen and it causes hydrogen embrittlement. On the other hand, when fuel cell reactions proceed at high temperatures, the increase in temperature causes metal diffusion in the hydrogen permeable metal layer. In a fuel cell with a hydrogen permeable metal layer, if hydrogen is supplied when the temperature of the hydrogen permeable metal layer is outside a certain temperature range, the hydrogen permeable metal layer degrades, potentially damaging the product life of the fuel cell.